The Federal Aviation Administration has ordered all American Airlines flights grounded in the U.S. after the carrier reported a technical issue affecting its entire system with millions traveling for the holiday.

In social media replies to frustrated travelers, the airline said: “Our team is currently working to rectify this. Your continued patience is appreciated.”

The company has not issued a press release explaining the issue and an email was not immediately returned.

A post on the Federal Aviation Administration’s website acknowledged the airline’s request for a “nationwide groundstop” for all American Airlines planes and their subsidiary airlines.

The groundings couldn’t come at a worse time for the millions of travelers expected to fly over the next 10 days.

The Transportation Security Administration expects to screen 40 million passengers over the holidays and through January 2.